In commercial aviation, the ability to accurately pinpoint an aircraft's position is important to safe and efficient air travel. Originally, pilots relied on visual cues to avoid obstacles during take-off and approach to landing. However, weather conditions often hinder the pilot's ability to see such objects. Consequently navigational procedures were developed to guide the aircraft into and out of terminal area which require only position information and not visual cues. Currently, airlines typically use ground based radio navigation systems to provide position information, particularly during poor visibility conditions. A disadvantage of ground-based radio positioning systems, however, is that such systems are not particularly accurate and provide less certainty of an aircraft's position the farther the aircraft is from the transmitter. Recognizing this limitation, regulators have established a set of criteria for building these navigational procedures called TERPS (Terminal Instrument Procedures) for designing approaches that recognize the limitations of the technology. TERPS employs trapezoidal obstacle identification surfaces that take into account inaccuracies in the aircraft's positional certainty. TERPS is formally defined in US FAA Order 8260.3B, along with associated documents in the 8260 series. The international equivalent of TERPS is called PANS-OPS, promulgated by the International Civil Aviation Organization (“ICAO”) (document 8168); the two combined represent virtually 100% of conventional approaches in place today. Such obstacle identification surfaces generally extend from the final approach fix, a point in space from which an approach begins, to a go-around decision altitude, or missed approach point. If a prospective obstacle identification surface would intersect an obstacle, the proposed surface (and therefore the flight path) must be offset or otherwise modified, which can result in the aircraft being in an undesirable position relative to the runway.
The missed approach point or decision altitude, in general terms, is the lowest point during an approach procedure wherein the obstacle identification surface clears all obstacles. If the aircraft landing conditions do not meet the requirements for a successful landing (e.g., visual contact with the runway environment, landing clearance, etc.), then the pilot makes a go-around decision and typically at the missed approach point the aircraft transitions to a missed approach surface that is similarly designed to provide for a safe extraction for a generic aircraft. In an obstacle rich environment, however, TERPS surfaces may not provide sufficient clearance to allow guidance all the way down to a decision altitude. In these cases, a non-precision approach is used that only provides guidance down to a particular minimum descent altitude. If the landing must be aborted below the minimum descent altitude, TERPS does not provide a missed approach surface. If an instrument approach is not available, the flight crew typically executes a circling procedure, which can present undue risk to the aircraft when conducted during low visibility. It is estimated that more than half of all aviation accidents involving controlled flights into terrain occur during such non-precision approaches, and that an aircraft is five times more likely to experience an incident during a non-precision approach.
Containment volumes (the protected volume enclosed by the obstacle identification surfaces) for traditional criteria sets such as TERPS and PANS-OPS have been established essentially through empirical analysis and experience and have been deemed safe due to the large number of operations that have been accomplished safely within these volumes. Navigation systems have improved by orders of magnitude over earlier technologies and permit much tighter containments than previously available. Public design criteria sets necessarily evolve slowly and have not kept up with these new navigation capabilities.
An alternative to TERPS for designing approaches is emerging, known as performance-based navigation. Under this concept, optimal flight paths are designed based on the aircraft's capabilities and not on the characteristics of the navigational signals. This permits advanced aircraft to execute advanced procedures and confers access, safety, efficiency, and capacity benefits to well-equipped aircraft. RNAV is a type of navigation that permits operation on any desired flight path (as opposed to point to point based on navigation beacons) within the limits of the available signals. Required Navigation Performance (“RNP”) is a term used to describe performance-based RNAV.
RNP is a new navigation method that requires a new means of understanding safety. In a sense, RNP inverts the safety function; instead of specifying the performance limitations of a particular navigational aid and then designing safe procedures around that, RNP procedures define the safe buffers required for an optimum procedure which in turn drives the requirements for the navigation system performance on the aircraft. In this way, procedures can be designed that are demonstrably safe, but can only be flown in aircraft that are known to possess sufficient navigation system accuracy and integrity. The essential question being answered by a conventional procedure is “what is the best way in, given the characteristics of the underlying navigational needs?”, whereas the essential question for an RNP procedure is “what level of performance is required to execute the safest and most efficient path to the runway?”
RNP is a statement of the navigation performance necessary for operation within a defined airspace. RNP navigation permits aircraft operation on any desired flight path, with clearly defined path specifications using navigation aids such as the global positioning system, and/or within the limits of the self-contained capability, such as inertial navigation systems. Modern systems are allowing carriers to transition from TERPS-based approach and landing procedures to more flexible linear surfaces developed using RNP, providing carriers with precision approach capability. A critical component of RNP is the ability of the aircraft navigation system to accurately monitor its achieved navigation performance and to ensure that it complies with the accuracy required for a specific route or airspace. It is estimated that 80% of the existing airline fleet is equipped with the flight management systems, navigation systems like DME, GPS, and INS, and the altimetry that is needed to implement RNP.
RNP-based approach and departure procedures provide important safety and performance benefits including the ability to complete a safe instrument approach on any available runway during poor visibility. Safety is enhanced by providing vertical guidance all the way through the entire procedure. Shorter, more direct routes are possible that save significant time and fuel. Airspace capacity is improved by permitting reduced separation standards for well-equipped aircraft. Air traffic control benefits from safe and predictable aircraft paths in both visual and instrument flight rule conditions, and the airports and airliners no longer need to rely on ground based landing systems.
There remains a need for improved methods for determining a safe corridor for aircraft approaching a landing that provides an efficient approach without negatively impacting acceptable levels of safety.